The present invention relates to digital input devices and, more particularly, to a method and system for digitizing handwriting while the user is writing.
Systems exist that are capable of digitizing the motion of a writing implement in real time. These systems fall into two broad categories: those that rely on a digitizing tablet, and those that do not. Examples of systems that rely on digitizing tablets include those described in U.S. Pat. No. 4,206,314 to Paden, U.S. Pat. No. 4,318,096 to Thornburg, U.S. Pat. No. 4,564,928 to Glenn, U.S. Pat. No. 4,641,354 to Fukunaga and U.S. Pat. No. 4,558,757 to Mori. These systems generally are suited to the real time digitization of handwriting; but they require the provision of a digitizing tablet, which often is inconvenient.
Systems that are independent of digitizing tablets generally lack sufficient accuracy for the digitization of handwriting. The system described by Epperson in U.S. Pat. No. 5,247,137 relies on an accelerometer-based inertial navigation system to track the writing implement. Because the position of the writing implement is determined by a double integration of the measured accelerations, this system is subject to an amount of drift that is unacceptably large for the present application. Other systems measure the coordinates of the writing implement directly, using one or another type of global positioning system. Systems based on propagation times of ultrasonic pulses are described by Hansen in U.S. Pat. No. 4,506,354; by Herrington, in U.S. Pat. No. 4,654,648; by the De Bruyne, in U.S. Pat. No. 4,758,691; by Mallicoat, in U.S. Pat. No. 4,777,329; by Sterfik, in U.S. Pat. No. 4,814,552; by Milner, in U.S. Pat. No. 4,862,152; by Gilehrist, in U.S. Pat. No. 4,991,148; by Edwards, in U.S. Pat. No. 5,142,506; and by Biggs, in U.S. Pat. No. 5,308,936. There are several reasons why these systems lack suitable resolution for the accurate rendition of handwriting. One is that these systems are subject to systematic errors caused by phenomena such as multiple travel paths of the ultrasonic signals. Another is that the ultrasonic signals are most conveniently generated in a frequency range, 40-60 KHz, in which the transmitted ultrasonic pulses are too long for adequate resolution. To achieve the higher frequencies and shorter pulses needed for requires high energy circuitry that cannot be packaged easily and safely in a writing implement. Suzuki describes an infrared-based system in U.S. Pat. No. 5,227,622, that suffers from the inconvenience of requiring a clear line of sight from the infrared transmitter on the writing implement to the external tracking device.
There is thus a widely recognized need for, and it would be highly advantageous to have, a system and method of digitizing handwriting in real time that is free of the limitations of the prior art systems.